1. Field of the Invention
The present invention relates generally to the fabrication of printed circuit boards and, more particularly, to the configuration of printed circuit board panels adapted for use in the automated assembly of printed circuit boards.
2. Description of the Prior Art
Printed circuit boards have been used for fabricating electronic circuit assemblies for decades. Such printed circuit boards consist of at least a single layer of patterned electrical conductors secured to a sheet of electrically insulating material. Most commonly, the pattern of the electrical conductors is created by etching away regions from a continuous layer of electrically conductive material that was previously secured to the sheet of insulating material. Also, a plurality of apertures frequently pierce both the layer of electrically conductive material and the sheet of insulating material so leads of electrical components may pass through the printed circuit board. After electrical components have been positioned on the printed circuit board with their leads passing through the board, solder is applied to both the electrically conductive printed circuit board patterns and the electrical component leads to form electrically conductive paths therebetween.
Over the years, increasingly more complicated printed circuit boards have been fabricated. Accordingly, standard commercial manufacture now provides multilayer printed circuit boards that combine into a single assembly multiple layers of different printed circuit patterns. Assembling these multilayer printed circuit boards requires fabricating the individual printed circuit pattern for each conductor layer and sandwiching the various conductor layers together into a one piece assembly. In these multilayer printed circuit boards, the different layers of conductive patterns are separated from one another by sheets of insulating material.
In addition to the apertures that pierce the printed circuit board to receive the leads of electrical components, multilayer printed circuit boards include apertures which generally pierce the conductor layers and intervening sheets of insulating material to provide electrical connections between different layers of patterned conductors. After the required apertures have been formed both for these inter-layer electrical connections and also for electrical component leads, the walls of the apertures are usually plated with an electrically conductive material thus electrically interconnecting conductor patterns on different conductive layers.
While a large fraction of printed circuit boards are assembled using component whose leads pass through the printed circuit board, increasing use is being made of a component attaching technique in which the leads do not pass through the printed circuit board. In this alternative technique instead of passing through the printed circuit board, the electrical component leads attach directly to conductor patterns on the outer surface of the printed circuit board. This alternative technique, which is called surface mounting, provides two distinct advantages over the other technique in which the electrical component leads pass through the printed circuit board. First, surface mounted components are frequently attached to both surfaces of the printed circuit board. Components are not generally mounted on both surfaces of the printed circuit boards where component leads pass through a printed circuit board because sharp ends of component leads project outward from the surface of the printed circuit board opposite to that on which the components are located. Secondly, the electrical leads of surface mounted components need not be as strong, and therefore can be much smaller, since they are not inserted through apertures in the printed circuit board.
The ability to use finer electrical leads for electrical components is particularly advantageous for modern complex and/or high-speed integrated circuit components. Individual complex integrated circuit components may require as many as one-hundred or more individual leads. The small leads that may be used with surface mounted components permits packaging complex integrated circuits in a smaller area on printed circuit boards. As for high-speed integrated circuit components, their operation may be adversely affected if they are connected to an electrical circuit by leads of differing lengths. The ability to use finer leads for high-speed integrated circuit components allows minimizing the difference in length among them thereby reducing as much as possible any adverse effect on the circuit's performance.
All of the preceding improvements in electronic circuit fabrication that result from the use of multilayer printed circuit boards in conjunction with surface mounted electronic components have necessitated corresponding advances in the techniques used to fabricate, assemble, and test printed circuit boards. Consequently, the fabrication and testing of printed circuit boards, which decades ago was entirely performed by hand, is now a highly automated process. Thus digital computers are typically used in laying-out the electrically conductive patterns, in preparing the artwork that is frequently used to fabricate those conductive patterns or in fabricating those patterns directly, in mechanically forming apertures through various layers of the multilayer printed circuit board both for inter-layer electrical connections and to receive any leads of electrical components that pass through the printed circuit board, in testing the printed circuit board before any electronic components are attached to it, in controlling the operation of automated equipment used to place electrical components on the printed circuit board, and in testing the operation of the assembled printed circuit board.
Because automated equipment is widely used in fabricating and assembling multilayer printed circuit boards, particularly those employing surface mounted components, industry has generally adopted a standard size, rectangularly shaped printed circuit board panel measuring 24 by 18 inches along its edges. Thus, automated processing equipment, adapted for this standard size printed circuit board panel, is generally used to fabricate any desired printed circuit board design that fits on this standard panel size.
In using this standard size printed circuit board panel, after a printed circuit board has been designed and its conductor patterns laid-out, multiple copies of the printed circuit board's patterns may be arranged at different locations on the standard sized panel. Thus, the processing of a single printed circuit board panel frequently produces several individual printed circuit board assemblies concurrently.
After a printed circuit board design has been laid out, copies arranged at different locations on the standard panel, and the printed circuit board panels fabricated and tested, they are ready for electrical component attachment. Component attachment is usually performed on an automated, computer controlled assembly line. For surface mounted components, the first step in this automated assembly process is printing solder paste onto one surface of the printed circuit board panel.
Solder paste is printed onto one surface of a printed circuit board panel through a stencil so the paste will be applied to only those areas of the conductive patterns to which electrical component leads are to be attached. After the stencil is installed on the assembly line, solder paste is automatically applied to one surface of a printed circuit board panel.
After the solder paste is applied, epoxy adhesive may also be applied under computer control to selected locations on the printed circuit board's surface. This epoxy adhesive bonds the electronic components, particularly surface mounted components, to one surface of the printed circuit board panel as they are automatically placed on the board by a computer controlled apparatus included in the automated assembly line.
After all the components have been placed on one surface of a printed circuit board panel, the surface is heated so the solder on the electrically conductive patterns of the printed circuit board panel will reflow together with solder on the leads of the electronic components. Reflowing the solder between the conductive patterns of the printed circuit board panel and the electronic component leads electrically and mechanically interconnects the components with the printed circuit board's electronic circuit.
If, after electronic components have been attached to one surface of a printed circuit board panel additional electronic components are to be attached to the other surface, then the automated, computer controlled assembly line must be set-up to process the second surface of the printed circuit board panel. Setting-up the automated assembly line again requires shutting the line down for as long as one hour to install a different solder paste stencil and re-program the computer-controlled assembly line to attach components to the second side of the panel. After the solder paste stencil has been installed and the equipment has been re-programmed, the automated assembly line may again operate in the manner described above to apply solder paste, apply epoxy adhesive, place electronic components, and reflow the solder for the second surface of the printed circuit board panel.
After all the electronic components have been attached to both surfaces of the printed circuit board panel, it is washed to remove excess solder paste and flux residue, cut-up or severed (depaneled) to separate all the individual printed circuit boards included in the panel, and then the individual printed circuit boards are tested electrically for proper operation.
Despite the extensive use of digital computers, the complexity of present printed circuit board manufacturing requires the use of relatively expensive tooling for fabricating, assembling, and testing each different multilayer printed circuit board design, particularly multilayer printed circuit board assemblies using surface mounted components attached to both surfaces of the printed circuit board. For example, tooling-up to fabricate such a 4 layer printed circuit board panel may cost over $1,000. In addition, fixtures for testing such a printed circuit board panel may cost as much as $10,000.
Even though the manufacture of multilayer printed circuit board assemblies in the manner described above is complex and requires expensive tooling, due to the complexity of such electronic assemblies and the difficulties involved in fabricating them by any other technique, it is economically practical to fabricate as few as 25 printed circuit board assemblies in the manner described above. Because tooling-up to manufacture only 25 printed circuit board assemblies may cost over $5,000, the tooling cost alone for manufacturing such a small quantity approaches $200 per printed circuit board assembly. Consequently, the cost for fabricating small quantities of multilayer printed circuit boards having surface mounted components attached to both surfaces of the printed circuit board is extremely sensitive to tooling charges. Accordingly, anything that can be done to reduce the expense of tooling to fabricate such printed circuit board assemblies and/or to reduce non-productive set-up time for the automated assembly line greatly reduces the cost of fabricating small quantities of such printed circuit board assemblies.
Another problem encountered when attaching components to both sides of a printed circuit board is that considerable work-in-process can accumulate. To reduce set-up time, typically all printed circuit boards in a production lot will have components attached to one side of the board, and then the lot of boards will have components attached to the other side. Due to this sequence of operations, no units will be fully assembled until all of the units have been half assembled.
This not only leads to a large volume of work-in-process, but also means that every board in the lot will be half assembled before any board is fully assembled and available for electrical testing. Such a lag between full assembly and testing adversely affects the yield of the assembly process.